Juno First Boot Leg

[NivagSwerdna]

Next we look at 1H going into the 74LS74 down by the 555... this should be used to delay 1H by one pixel clock...

First... 1H on pin 2...



Then the output on pin 6...

That output looks pretty anaemic.. and it looks just like the input... not sure that was the intent here... something to ponder on...

 

[NivagSwerdna]

Added a 100pF capacitor between p3 and p7 of I12...



Signal changes...


Oh dear... that is not really as intended... the upper trace which is E has now doubled in frequency! This should have been unaffected and be 1.5MHz for the 6809E... 3MHz is clearly wrong.

 

[NivagSwerdna]

Happy New Year!

So it's either throw a tantrum and put this in the bin or have another go...



So here we are in the section that is pretending to be a Konami 081 Dynamic RAM custom...
CH0 CLK, CH1 MXA, CH2 MXB, CH3 E, CH4 Q, CH5 P, CH6 CAS CH7 RAS..


So it's worse than it was! MXB, E and Q are now flat lined. The pixel clock P has the wrong duty cycle.
Back to basics I suppose!

 

[NivagSwerdna]

Removed the 74LS161 which had a small cap on pin 7 to GND...



So we only care about CH5... the Pixel Clock... this is effectively CLK / 3 i.e. 6MHz and now the duty cycle is correct. That cap is obviously far too large.. I'm going to remove it and replace the 161 for good measure...

 

[NivagSwerdna]

Replaced with 74HCT161... no caps...

CH5 looks OK... CH3 E and CH4 Q are 2x fast... should be 1.5MHz for the 6809E

I think the schematic looks something like this...

given H15 and H16 are clocked on +ve edges but B is not CLK... I would expect E the output of pin 13 of H16 to look more like...

i.e. Only count when CH5 is High and could on -ve edge or something like that... but that looks like it would be too slow... anyway... time to look close at H16

 

[NivagSwerdna]

Without the novelty of trying to fix this board I would probably be out on the streets robbing old grannies or smashing up bus shelters or something so probably just best to stick with it for a while longer...

Looking at the 2nd 74LS161... H16...



So this is fed from pin 15 of H15 which is Ripple Carry Out... i.e. Carry... H15 is counting... E, F, 0, E, F, 0, ... with carry on the 0...

Here's H16...


It's all over the place! Something not good here. (NOTE: I have removed all the lurking small caps at this stage).
I really need pin 13 to be the pixel clock 6MHz divided by 4 to give the 6809 E clock... but here it's a mess and looking more like 3MHz...
Now I'm going to go very off piste here and say I think the design is wrong!

Firstly H16 is driven from the NOT of the H15 clock (from pin 12 of the 74LS04), so it is 180 degrees out of phase with H15.. But look at the signals that drive it...



74LS161 is positive edge triggered... so there is a count when CH6 (aka pin 7 (and pin 10)) are HI but at the trailing edge of CH6 (which is the Pixel Clock) there is a point at which the edges are VERY close together. I think that's a problem! Now people cleverer than me seem to know how to fix that with small caps but I don't so I think I'm going to do something very drastic... I'm going to cut the trace and drive H16 with the SAME clock as H15.
My reasoning is that the RCO (aka TC aka pin 15) output has long enough propagation, according to datasheet 23ns typical, that we end up dangerously close to the rising edge of the NOTed clock; by using the other phase we are going to be safer (although one half clock delayed)...
I wonder how this is going to work out... let me know if you think I've lost the plot!

 

[NivagSwerdna]

Here's the plan...

Will try and find some time tomorrow evening and see what damage I have done!

 

[NivagSwerdna]

Bodging done... no time to test tonight... but soon... now both H15 and H16 are clocked by the output of pin 13 (connected to pin 2) of the neighbouring 74LS04...

Place your bets...

(off to watch Traitors)

 

[NivagSwerdna]

Well it could be worse...


So CH12 which is pin 13... that is 1.5MHz for the E of the 6809E... that looks really good...
But CH13 which is pin 14... should be 3MHz... but it is glitchy....



It's like it's trying to be 3MHz but has glitches... however... it's a counter and after the glitch it seems to count correctly... i.e. we are not getting extra counts, we are just getting glitches in the output

Will have to think about that... is it something downstream causing issues?

 

[NivagSwerdna]

Limped into 2025 and then got RSV/Flu so have been out of commission and too sick to do anything but today did a quick scope..

So this is pin 13 of H16 which should be a 1.5MHz signal (which becomes E for the 6809E)...


Now that's very cool.
Now looking at pin 14 which showed up as glitchy on the logic analyser...


So that is sort of right... it is 3 MHz... but it's really not very square at all.
If I lift the leg out of the socket and re-measure...

It looks good... just struggles to drive things downstream... I guess I should think about what it drives and see if I can make it squarer.

 

[TheDaddy]

I will be honest I would have chucked this on the ' 2 Hard Pile ' a long time ago ! Hats of too you for your persistence !

Dave.

 

[NivagSwerdna]

OK... so the flu/plague/RSV or whatever I had is receeding... I have this board balanced on a box in my very small work area so I guess I have to keep looking...

So I removed the 74LS00 at G15 which is downstream of the 74LS161 pin 14 in an attempt to get a nicer looking output. I read that a 74LS161 has a large fan out so it is odd that this signal is not more square on my scope..


measuring...



gives..



So... that's no better... OK... what to remove next...

 

[NivagSwerdna]

Going nuts here... let's take a step back... bend up pins 13 and 14...


And measure...
Pin 13...

That's good... 1.5MHz nice and square...

Pin 14..


3MHz and square if you use your imagination...
Must be something downstream which is loading it down...
Sadly I have domestic duties... so will have to wait until later in the week... but I'm going to find this!

 

[bones]

Your persistence is truly astounding, always a good read,and a good demonstration of the order with which to approach a repair. Top stuff!!
When you are finished with this I have a moon patrol bootleg stack that I got from Paulcan69 many years ago that has no sockets populated at all, and I'm never going to be able dove into that one lol.

 

[NivagSwerdna]

When you are finished with this I have a moon patrol bootleg stack that I got from Paulcan69 many years ago that has no sockets populated at all, and I'm never going to be able dove into that one lol.

I try and stick with early Konami or Galaxian and variants... I think I understand most of that genre... So any unfixable Konami... However everything is on hold until I have fixed this b***ard!

 

[bones]

Are you sure ? So many rusty legs, dodgy sockets,40+ year old ribbon cables and gazillions of fujitsu chips......... a dream of a challenge lol.

 

[rotunda]

I recently discovered this game. It's bloody fantastic! Great little shooter.

 

[NivagSwerdna]

I am a bit busy this week so probably cannot have a proper go but I think I have found why the signal integrity on the counter output is being meddled with. The bootleggers seem to have added a small cap on the pair of 74LS373 input. This is not on the original schematic and I think it is not really good idea... It's going to go.
On the board they have bodged the timings in a number of places... Hope to fix that.

 

[NivagSwerdna]

OK... this is getting so close to going in the bin... I am frustrated and tired and managed to bugger up some traces removing a device last night...




What a moron I am. The middle trace needs a bodge wire. I'll have to use the microscope for that and tidy up first to get some space on the desk!

This is very painful.

Moral of the story is don't try a quick fix after a long day at work.

 

[Bods]

I seem to take ages to do stuff these days, soon as I try and crack on speeding up I make some stupid mistake

Been there myself with doing things after work